1. Field of the Invention
The invention relates to an optical communications device. More particularly, the invention pertains to photonic crystal optical demultiplexer devices produced from a three-dimensionally-periodic, porous, dielectric, photonic crystalline structure, which has surfaces or interfaces that are inverse replicas of the surfaces of a monodispersed sphere array.
2. Technical Background
Optical fibers are key components in integrated optical circuits that route and control optical signals in optical communication systems In such optical communication systems, information is transmitted at infrared optical frequencies by carrier waves that are generated by light sources such as lasers and light-emitting diodes. These optical communication systems offer several advantages over traditional electronic communications systems using copper wires or coaxial cables. They have a greatly increased number of channels of communication, as well as the ability to transmit messages at much higher speeds than the electronic systems.
Optical fibers are thin strands of glass or polymer capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. Communication systems now increasingly employ such optical fibers because of their high speed, low attenuation and wide bandwidth characteristics, which can be used for carrying data, video and voice signals concurrently. In essence, an optical fiber is a small diameter waveguide and light rays are guided along the axis of the fiber with minimum attenuation.
Modern fiber optic communication systems have the ability to simultaneously transfer light signals having differing wavelengths over a single optical fiber. An important part of these communication systems is the use of wavelength division multiplexing, by which a given wavelength band is segmented into separate wavelengths so that multiple signals can be simultaneously carried on a single installed line. A typical optical fiber communications system comprises a source of optical input signals, a length of optical fiber coupled to the source and a receiver for optical signals coupled to the fiber. In multi-wavelength systems a plurality of nodes may be provided along the fiber for adding or dropping wavelength channels.
Multi-wavelength systems use multiplexers and demultiplexers which are capable of dividing the band into given multiples of different wavelengths which are separate but closely spaced. Adding individual wavelengths to a wideband signal, and extracting a given wavelength from a multi-wavelength signal, requires wavelength selective couplers and demultiplexers for selectively controlling specific wavelengths of light. Demultiplexers are advantageous for differentiating and manipulating such optical signals based on their wavelength.
Recent developments in the field of wavelength division multiplexing technology have resulted in increased data bandwidth, an improvement by over two orders of magnitude, over a single optical fiber. These developments have created an increased demand for improved wavelength multiplexer, demultiplexer, and add-drop filters. It would be desired to produce low-cost, compact and high-performance optical demultiplexers for optical communications. An important class of such photonic devices includes photonic band gap structures, also known as photonic crystals. It has now been found that photonic band gap devices can be used to produce low-cost, efficient, and compact multiplexers, demultiplexers, and add-drop filters, thereby permitting the expansion of wavelength division multiplexing technologies from the presently existing telecommunication applications to data communications.
Photonic band gap structures are periodic dielectric structures that can confine and control light in three dimensions. Photonic band gap devices are crystals with periodic structures that, through interference, prevent certain electromagnetic waves from propagating through them. These structures exhibit a photonic bandgap analogous to the electronic bandgap in semiconductors. If the dielectric constants of the materials are different enough, i.e. high contrast, and the absorption of light by the materials is minimal, then scattering at the crystal interfaces can produce many of the same phenomena for photons as the atomic potential does for electrons. Hence it is possible to construct photonic crystals which will reflect light of a chosen wavelength while other longer or shorter wavelengths are transmitted. Light that has a wavelength that lies in the bandgap is prevented from existing inside the crystal and hence is reflected by the crystal. Heretofore, photonic bandgap devices in the wavelength ranges of interest for optical communications (1-2 xcexcm) have not been produced because the critical dimensions of a structure with the wavelength of the light that is being manipulated and a reduction in the wavelength to the 1-2 xcexcm range necessitates the fabrication of structures with minimum feature sizes that are in the micron to sub-micron range.
The photonic crystal optical demultiplexers of this invention comprise a photonic crystalline structure positioned between of optical waveguides or fibers including an input fiber and one or more output fibers. The first optical waveguide is positioned to direct a broad wavelength band of incident light onto the crystalline structure and the second optical waveguide is positioned to receive a narrow wavelength band of reflected light from the crystalline structure. Alternatively, the first optical waveguide is positioned to direct a broad wavelength band of incident light onto the crystalline structure and the second optical waveguide is positioned to receive one or more narrow bands of light refracted through the crystalline structure in a prismatic fashion.
The invention provides a photonic bandgap demultiplexer which comprises:
a) a plurality of three-dimensionally-periodic, porous, dielectric, photonic crystalline structures, which structures have surfaces or interfaces that are inverse replicas of the surfaces of a monodispersed sphere array, wherein necks exists between neighboring spheres in said sphere array and the average sphere diameter does not exceed about 1000 nm;
b) a plurality of first optical waveguides, each of the first optical waveguides positioned to direct a broad wavelength band of incident light onto one of the crystalline structures; and
c) a plurality of second optical waveguides, each of the second optical waveguides positioned to receive a narrow wavelength band of light reflected from or refracted from one of the crystalline structures.
The invention also provides a photonic crystal optical demultiplexer which comprises:
a) a three-dimensionally-periodic, porous, dielectric, photonic crystalline structure, which structure has surfaces or interfaces that are inverse replicas of the surfaces of a monodispersed sphere array, wherein necks exists between neighboring spheres in said sphere array and the average sphere diameter does not exceed about 1000 nm;
b) a first optical waveguide positioned to direct a broad wavelength band of incident light onto the crystalline structure; and
c) one or more second optical waveguides positioned to receive a narrow wavelength band of light reflected from or refracted through the crystalline structure.
The invention further provides process for preparing a three-dimensionally-periodic, porous, dielectric, photonic crystalline structure which comprises forming an array of microscopic spheres on a smooth substrate into a face centered cubic structure having spaces between adjacent spheres, which spheres have an average diameter not exceeding about 1000 nm; sintering the spheres under conditions sufficient to attach adjacent spheres to one another by an intermediate neck; forming a solid silicon structure in the spaces between adjacent spheres by infiltrating silane gas into the spaces in a low pressure chemical vapor deposition process; wherein the silane gas is infiltrated into the spaces at a temperature of from about 450xc2x0 C. to about 600xc2x0 C., at a pressure of from about 100 mtorr to about 600 mtorr and at a flow rate of from about 50 sccm/min. to about 150 sccm/min., and then removing the spheres.